With development of a photovoltaic industry, a system voltage of a photovoltaic assembly increases from 1000V to 1500V. In order to fit a photovoltaic assembly with a system voltage being 1500V, an inverter connected to the photovoltaic assembly generally adopts a five level topology structure.
When the inverter with the five level topology structure normally operates, the inverter operates in a five level operation mode, that is, the inverter alternately outputs five different levels in different switching state combinations of switching transistors. As shown in FIG. 1, a floating capacitor voltage Ufc is Udc/4 when the inverter with the five level topology structure normally operates. For an output voltage UAN of the inverter, the output voltage has five levels respectively being +Udc/2 (S1 and S7 are switched on), +Udc/4 (S1 and S8 are switched on or S4, S5 and S7 are switched on), 0 (S2, S3 and S7 are switched on or S4, S5 and S8 are switched on), −Udc/4 (S6 and S7 are switched on or S2, S3 and S8 are switched on) and −Udc/2 (S6 and S8 are switched on). S1, S2, S3, S4, S5, S6, S7 and S8 are all switching transistors, Udc is an operation voltage setting threshold when the inverter with the five level topology structure normally operates, Udc is less than 1500V and Udc generally ranges from 800V to 1300V.
However, when the inverter starts operation, a direct current side voltage of the inverter is an open-circuit voltage which is 1500V, in order to avoid voltage stresses of the switching transistors S1 and S6 exceeding a withstanding range, the floating capacitor voltage Ufc is pre-charged to a preset value (such as 450V). In this case, the inverter alternately outputs seven different levels in different switching state combinations of switching transistors. For example, the floating capacitor voltage Ufc is pre-charged to 450V that is 3Udc′/10, the output voltage UAN of the inverter in FIG. 1 may have seven levels respectively being +Udc′/2 (S1 and S7 are switched on), +Udc′/5 (S1 and S8 are switched on), +3Udc′/10 (S4, S5 and S7 are switched on), 0 (S2, S3 and S7 are switched on or S4, S5 and S8 are switched on), −2Udc′/10 (S6 and S7 are switched on), −3Udc′/10 (S2, S3 and S8 are switched on) and −Udc′/2 (S6 and S8 are switched on), Udc′ is 1500V. In this case, if the inverter is still controlled to operate in the five level operation mode, an output current of the inverter may be severely distorted, thereby affecting an operation stability of a whole photovoltaic system.